STL rework

- fixed lots of bugs
- overlay libraries work correctly and reliably
- the STL is an overlay library
- examples updated

src/stl/str.rs

@@ -0,0 +1,39 @@
+use super::litconv::{with_str, with_uint};
+use super::RuntimeError;
+use crate::interner::Interner;
+use crate::pipeline::ConstTree;
+use crate::{define_fn, Literal};
+
+define_fn! {expr=x in
+  /// Append a string to another
+  pub Concatenate {
+    a: String as with_str(x, |s| Ok(s.clone())),
+    b: String as with_str(x, |s| Ok(s.clone()))
+  } => Ok(Literal::Str(a.to_owned() + b).into())
+}
+
+define_fn! {expr=x in
+  pub CharAt {
+    s: String as with_str(x, |s| Ok(s.clone())),
+    i: u64 as with_uint(x, Ok)
+  } => {
+    if let Some(c) = s.chars().nth(*i as usize) {
+      Ok(Literal::Char(c).into())
+    } else {
+      RuntimeError::fail(
+        "Character index out of bounds".to_string(),
+        "indexing string",
+      )?
+    }
+  }
+}
+
+pub fn str(i: &Interner) -> ConstTree {
+  ConstTree::tree([(
+    i.i("str"),
+    ConstTree::tree([
+      (i.i("concatenate"), ConstTree::xfn(Concatenate)),
+      (i.i("char_at"), ConstTree::xfn(CharAt)),
+    ]),
+  )])
+}